In the large district in the southwest of the city, rebels had mounted a machinegun on a red pick-up truck with the words "United Mujahedeen Brigade" spray-painted on its side.

A video posted online by activists shows them shooting frantically at the helicopter gunships above, spurred on by the rallying cries of other rebels in the street.

Another video shows flames licking from a building as gunfire crackles in the background and verses from the Koran can be heard from a nearby mosque.

Women and children have been evacuating the Salaheddin area since Friday, seeking refuge in other areas of the city. Those residents who have opted to stay behind have sought refuge in the basements of houses.

According to the AFP correspondent, there is no electricity and water in the city, and food stocks are so low that it is nearly impossible to find bread supplies.

"There are thousands of people in the streets fleeing the bombardment. They're being terrorised by helicopter gunships flying at low altitude," said one activist who identified himself as Amer.

"There's a large number of civilians who have taken refuge in public parks in safer areas, but most took refuge in schools," he told AFP via Skype.

"They cannot get out of town and there is no safe place left for them in Syria," he added.

Until recently, both Aleppo and the capital Damascus had been considered to be relative safe havens.

A Syrian security source told AFP: "Hotspots have been completely blocked off to stop the terrorists from escaping," the term the regime uses when it refers to the rebels.

One militant suggested that regime forces were resorting to subterfuge.

"Some soldiers are passing themselves off as rebels, and putting up checkpoints with the rebel flag to trap fighters taking supplies between secure areas and 'hot' districts," he said.